Methods and Apparatus for Acoustic Backscatter Communication

ABSTRACT

A communication system may communicate by backscattered acoustic signals that propagate through a liquid or solid. In this system, one or more transmitters may transmit acoustic signals that travel to, and are reflected by, an acoustic backscatter node. The backscatter node may modulate the amplitude and/or phase of the reflected acoustic signals, by varying the acoustic reflectance of a piezoelectric transducer onboard the node. The modulated signals that reflect from the backscatter node may travel to a microphone and may be decoded. The backscatter node may include sensors, and the uplink signals may encode sensor readings. The backscatter node may harvest energy from the downlink acoustic signals, enabling the node and the sensors to be battery-free. Multiple backscatter nodes may communicate concurrently at different acoustic frequencies. To achieve this, each node may have a matching circuit with a different resonant frequency.

RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No.62/850,416 filed May 20, 2019 (the “Provisional”).

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with government support under grant number029316-00001 awarded by the Office of Naval Research. The U.S.government has certain rights in the invention.

FIELD OF TECHNOLOGY

The present invention relates generally to acoustic backscattercommunication.

SUMMARY

In illustrative implementations of this invention, an acousticbackscatter network enables near-zero power communication through aliquid or solid medium.

For instance, a transmitter may transmit acoustic signals comprisingsound in the frequency range of 1 Hz to 20 kHz. These acoustic signalsmay propagate through a liquid or solid medium until they reach abackscatter node. The backscatter node may modulate the amplitude and/orphase of the acoustic signals in such a way that the resulting modulatedacoustic signal: (a) encodes data and (b) reflects from the backscatternode. The modulated acoustic signals may travel through the liquid orsolid medium to a microphone. The microphone may measure the modulatedacoustic signals. A computer may analyze these measurements to decodethe data.

In some cases, the acoustic backscatter node harvests energy from theacoustic signals and does not include a battery. In these cases, thepower consumption of the backscatter node may be near-zero, because thebackscatter node merely reflects (and modulates) acoustic signals, anddoes not actively generate acoustic signals.

In some cases, the backscatter node includes a piezoelectric transducer.The backscatter node may modulate amplitude and/or phase of acousticsignals that reflect from the piezoelectric transducer. To do so, thebackscatter node may control the acoustic reflectance of the transducer,by controlling the electrical impedance of an electrical load that iselectrically connected to the transducer. Specifically, decreasing theload impedance (e.g., by shorting the two terminals of the piezoelectrictransducer) may tend to stiffen the piezoelectric transducer, therebyreducing the strain (displacement) that the transducer undergoes inresponse to incident acoustic signals, thereby making the transducermore acoustically reflective. Likewise, increasing the load impedancemay tend to make the piezoelectric transducer more flexible, therebyincreasing the strain that the piezoelectric transducer undergoes inresponse to incident acoustic signals, thereby making the transducerless acoustically reflective (or equivalently, more acousticallyabsorptive).

In some cases, multiple backscatter nodes communicate concurrently overdifferent frequency channels. Multiple acoustic speakers may transmitmultiple acoustic signals concurrently, each at a different acousticfrequency. There may be multiple backscatter nodes, each with adifferent acoustic resonant frequency. The different resonantfrequencies of the backscatter nodes may match the respectivefrequencies at which the speakers are transmitting. Put differently,each backscatter node may be tuned to one of the multiple frequencies.For instance, to tune a backscatter node to a desired acousticfrequency, the backscatter node may include an RLC(resistor-inductor-capacitor) circuit with a resonant frequency that isthe same as the desired frequency.

In some cases, the system employs a MAC (medium access control) protocolfor concurrent communications by different backscatter nodes overdifferent frequency channels. For instance, an FDMA (frequency-divisionmultiple access) protocol may be employed. A MIMO (multiple-input andmultiple-output) decoder may be used to decode the different signals,despite signal collisions.

In some cases: (a) a communication system includes multiple acousticbackscatter nodes; (b) each backscatter node in the system includes oneor more sensors; and (c) the backscatter networking enables multiplebackscatter nodes to concurrently transmit sensor readings at differentacoustic frequencies

In some cases, a single backscatter node operates at only a singlepre-defined resonant frequency. Alternatively, a backscatter node may beable to change its own resonant frequency. To do so, the backscatternode may include multiple matching circuits, each of which has adifferent resonant frequency. The backscatter node may tune its ownresonant frequency by switching which of these onboard matching circuitsis being used.

As noted above, the backscatter node may be battery-free and may harvestenergy from incident acoustic signals. The harvested energy may be usedto power a microcontroller onboard the backscatter node. In some cases,the harvested energy is also used to power one or more sensors that areelectrically connected to the backscatter node. Each backscatter nodemay include energy-harvesting hardware, including a piezoelectrictransducer, multi-stage rectifier, and supercapacitor. When the acousticreflectance of the piezoelectric transducer is low or zero, thepiezoelectric transducer may convert time-varying acoustic signals thatare incident on the transducer into time-varying electrical voltage. Themulti-stage rectifier may convert this time-varying voltage into DC(direct current) voltage and amplify it. The supercapacitor may thenstore electrical energy.

Load matching may employed, to ensure efficient harvesting of energy.When the backscatter node is harvesting energy, the output electricalimpedance of the piezoelectric transducer onboard the node may be equalto the complex conjugate of the input electrical impedance of anelectrical load that is also onboard the node and is electricallyattached to the transducer. For instance, this electrical load mayinclude a matching circuit.

In some cases, the acoustic signals are sound that propagates through aliquid medium, such as: (a) ocean water or (b) fresh water of a river orlake. For instance, in a non-limiting use scenario of this invention, abackscatter node powers up by harvesting energy from acoustic signalsthat are transmitted through 10 meters of ocean water, and then reflectsmodulated acoustic signals that travel through 10 meters of ocean waterto a hydrophone, achieving a data throughput of about 3 kilobytes persecond.

Alternatively, in some cases, the acoustic signals are sound thatpropagates through a solid medium, such as metal, wood, gypsum, drywall,plaster or cement. For instance, the acoustic signals may travel througha metal pipe or a wall, floor or ceiling of a building.

In some cases, the system employs a communication protocol in which thebackscatter node is treated as having binary acoustic reflection states:0 and 1. The 0 and 1 states may correspond to low and high amplitudeacoustic signals, respectively, that reflect from the backscatter node.Or, for instance, the 0 and 1 states may correspond to different phases(or different ranges of phases) of acoustic signals that reflect fromthe backscatter node. In this approach with binary reflection states,each bit of data that is encoded in a modulated acoustic signal (whichreflects from the backscatter node) may correspond to a reflection stateor to a transition (e.g., rising edge or falling edge of amplitude)between reflection states.

In some cases, the system employs a communication protocol in which thebackscatter node is treated as having more than two acoustic reflectionstates, each of which is determined by amplitude and phase of sound thatreflects from the backscatter node. Put differently, each reflectionstate may be a pair of amplitude and phase, where the amplitude is anamplitude of reflected sound within a specified range of amplitudes andthe phase is a phase of reflected sound within a specified range ofphases. For instance, in some cases: (a) the system is treated as havingfour reflection states (each of which is a pair of amplitude and phase);and (b) two bits of data are encoded by a reflection state or by atransition between reflection states. In other cases: (a) the system istreated as having eight reflection states (each of which is a pair ofamplitude and phase); and (b) three bits of data are encoded by areflection state or by a transition between reflection states.

In some cases, the backscatter node modulates amplitude and/or phase ofacoustic signals that reflect from the backscatter node. Thus, in somecases, the system may employ an encoding scheme that modulates phase oramplitude of a carrier signal, such as ASK (amplitude-shift keying), QAM(quadrature amplitude modulation) or PSK (phase-shift keying).

This invention has many practical applications. For instance, in somecases, this invention may be employed for near-zero power acousticcommunication with underwater sensors that are used for underwaterclimate change studies, studying marine life, or oil exploration. Or,for instance, this invention may be employed for acoustic communicationwith remote sensors via acoustic signals that travel through solidmaterials (e.g. through solid pipes for oil exploration, or throughsolid walls for military or security purposes). More generally, thisinvention may be employed to advantage in any long-term deployment ofvery-low power sensors that communicate their readings over longdistances (e.g., meters, tens of meters, hundreds of meters, orkilometers) through a liquid or solid medium.

The Summary and Abstract sections and the title of this document: (a) donot limit this invention; (b) are intended only to give a generalintroduction to some illustrative implementations of this invention; (c)do not describe all of the details of this invention; and (d) merelydescribe non-limiting examples of this invention. This invention may beimplemented in many other ways. Likewise, the Field of Technologysection is not limiting; instead it identifies, in a general,non-exclusive manner, a field of technology to which someimplementations of this invention generally relate.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a system for acoustic backscatter communication.

FIG. 2 is an exploded view of a piezoelectric transducer.

FIG. 3 is a circuit diagram for an acoustic backscatter node that iswell-suited for underwater backscatter communication.

FIGS. 4A and 4B illustrate electrical circuits for encoding one bit ofdata per acoustic reflection state.

FIG. 5 illustrates an electrical circuit for encoding two or more bitsof data per acoustic reflection state.

FIG. 6 illustrates a system in which acoustic backscatter is employed tocommunicate concurrently over different frequency channels.

The above Figures are not necessarily drawn to scale. The above Figuresshow illustrative implementations of this invention, or provideinformation that relates to those implementations. The examples shown inthe above Figures do not limit this invention. This invention may beimplemented in many other ways.

DETAILED DESCRIPTION

Acoustic Backscatter Network

FIG. 1 shows a system for acoustic backscatter communication. In FIG. 1,a set of one or more acoustic transmitters 111 transmits acousticsignals. These acoustic signals 131 are sound waves that propagatethrough a liquid or solid medium 150, such as ocean water, fresh water,metal, wood, gypsum or cement. The acoustic signals 131 travel to anacoustic backscatter node 100. The backscatter node 100 modulates itsacoustic reflectance, in such a way as to modify amplitude and/or phaseof the acoustic signals that reflect from the backscatter node. Theresulting modulated acoustic signals 141 propagate through the liquid orsolid medium 150 to a microphone 112.

In some implementations: (a) the acoustic signals propagate throughwater; and (b) microphone 112 is a hydrophone. The hydrophone mayinclude a piezoelectric transducer that converts an acoustic signal intoan analog electrical signal. More generally, in some cases, microphone112 is a dynamic microphone (e.g., with a coil of wire suspended in amagnetic field), a condenser microphone (e.g., which employs a vibratingdiaphragm as a capacitor plate), or a piezoelectric microphone. Themicrophone may include a preamplifier.

In some cases, the acoustic transmitters 111 are piezoelectricunderwater speakers that transmit acoustic signals through water. Theacoustic transmitters may convert an electrical signal into sound.

In some cases, all or part of the communication system is immersedinside, or surrounded by, or in direct physical contact with, the liquidor solid medium. For instance, in some cases, the transmitter(s) 111,backscatter node 100, and a hydrophone (microphone 112) are all immersedin ocean water or fresh water. Or, for instance, transmitter(s) 111,backscatter node 100 and microphone 112 may each be external to, and maytouch, a solid medium (such as a pipe or a building wall).

In FIG. 1, the backscatter node includes a piezoelectric transducer 101,matching circuit 102 with a controllable impedance, energy harvester103, and microcontroller (MCU) 104. The microcontroller 104 may controlthe electrical impedance of the matching circuit 102, such as by causingone or more transistor switches in the circuit to open or close. Bycontrolling this electrical impedance, the microcontroller may controlthe acoustic reflectance of the piezoelectric transducer, and thus maycontrol the amplitude and/or phase of acoustic signals that reflect fromthe piezoelectric transducer.

When the piezoelectric transducer is in an acoustically non-reflective(or low reflective) state, the piezoelectric transducer may absorbenergy from the acoustic signals, converting the acoustic signals into atime-varying electrical voltage. The energy harvester 103 may include amulti-stage rectifier that rectifies and amplifies this voltage. Theenergy harvester 103 may also include a supercapacitor that storeselectrical energy.

Computer 120 may control and interface with the transmitter(s) 111 andthe microphone 112. In some cases, computer 120 comprises a controller,microcontroller or microprocessor.

Computer 120 may cause the acoustic transmitter(s) 111 to initiallytransmit a continuous, steady-state acoustic signal. The backscatternode 100 may harvest energy from this signal. Computer 120 may cause thetransmitter(s) 111 to send an acoustic query signal to initiate acousticcommunication with backscatter node 100. For instance, pulse widthmodulation (PWM) may encode bits in the query signal, in such a way thatdifferent length gaps between acoustic pulses may encode different bits(e.g., 0 and 1). The piezoelectric transducer 101 onboard thebackscatter node 100 may convert the acoustic query signal into anelectrical signal. This electrical signal, in turn, may be decoded bymicrocontroller 104 onboard the backscatter node. Once microcontroller104 has successfully decoded the query signal, microcontroller 104 maycause the backscatter node to modulate acoustic signals that arereflecting from the node, and to thereby send modulated acoustic signalsto the microphone 112. These modulated acoustic signals may encode thebackscatter node's preamble followed by other data (e.g., data collectedfrom a sensor). In some cases, multiple backscatter nodes are reflectingsignals to the microphone concurrently. The preambles of the respectivebackscatter nodes may be known by computer 120. By correlating, thecomputer may identify the respective preambles and thus the respectivebackscatter nodes that are sending signals. The backscattered signal maybe encoded using an FM0 scheme, Manchester scheme or other modulationscheme.

In illustrative implementations, the backscatter node includes apiezoelectric transducer. In some cases, the piezoelectric material inthis transducer comprises a PZT polycrystalline ceramic. (PZT is a solidsolution of PbZrO₃ and PbTi O₃). Or, for instance, the piezoelectricmaterial may comprise a single crystal, such as PZN-PT single crystal(i.e., a single crystal of a solid solution of lead zinc niobate andlead titanate) or a PMN-PT single crystal (i.e., a single crystal of asolid solution of lead magnesium niobate and lead titanate). Or, forinstance, the piezoelectric material may comprise a piezoelectriccomposite or a piezoelectric polymer.

In some cases, the piezoelectric transducer is air-backed and partiallypotted. In other words, the piezoelectric material may be encapsulatedby a polymer capsule in such a way that air pockets partially surroundthe piezoelectric material inside the polymer encapsulation. Forinstance, the polymer capsule may prevent water from entering thetransducer. Alternatively, the piezoelectric transducer may be fullypotted (i.e., without any air pockets inside the polymer capsule).

The piezoelectric transducer may enable backscatter communication, bymodulating sound waves that reflect from the transducer. In some cases,the modulation involves switching between reflective and non-reflectivestates of the transducer. For instance, the piezoelectric transducer maytransmit a “0” bit by absorbing all (or a large portion) of the incomingenergy, and may transmit a “1” bit by reflecting all (or a large portionof) the impinging acoustic signal. The transducer may switch between thereflective and absorptive states by modulating the voltage across thepiezoelectric interface, which in turn determines its vibrationamplitude (i.e., reflection). The microphone may receive the modulatedacoustic signals and sense changes in the amplitude due to reflection. Amicrocontroller may decode these changes to recover the transmittedmessages.

The backscatter node may cause the piezoelectric material to operate asa reflector by preventing the piezoelectric material from deforming(i.e., reducing or nulling the strain). Reducing the strain (i.e.,reducing deformation of the piezoelectric material) tends to prevent thematerial from absorbing the incoming acoustic signal and thus tends tocause the material to reflect that signal. To reduce the deformation,the backscatter node may activate a transistor switch to electricallyshort two terminal electrodes of the piezoelectric transducer. Suchswitching may require near-zero power and may be done entirely using theharvested energy, enabling the backscatter node to be battery-free.

In some cases, in order to backscatter an incoming acoustic signal, thebackscatter node turns on a switch that connects the two terminals ofthe piezoelectric device. Doing so may ensure that the chargedistribution and the electric field are both set to zero in the steadystate (since there is no voltage or charge when the terminals areshorted).

The total deformation (strain) of a piezoelectric material may bemodeled as:

S=s ^(E) T+dE

where S is strain applied on the material, E is an electric fieldapplied on the material, and s^(E) is the compliance coefficient underconstant electric field.

In some cases, when the piezoelectric material is in reflective state,both the electric field and the net tensor are zero. This means that thetotal strain is nulled (or almost nulled). In other words, thebackscatter node may transform the piezoelectric material into areflector by preventing it from deforming. This may cause the materialto reflect all (or a large portion) of the power of an incoming pressurewave.

The preceding discussion focuses on switching between two reflectivestates to enable backscatter, and on treating the two states as bits of“0” and “1”. In practice, backscatter communication may be made morerobust by adopting modulation schemes such as FM0 or Manchesterencoding, where the reflection state switches at every bit, enabling thereceiver to better delineate the bits and robustly decode backscattersignals. Hence, in some cases, the backscatter node employs FM0modulation for uplink communication (i.e., for communication viamodulated acoustic signals that reflect from the backscatter node andtravel to the microphone).

The piezoelectric transducer onboard a backscatter node may help harvestenergy from a downlink acoustic signal (i.e., from an acoustic signalthat is transmitted by an acoustic transmitter and that travels to thenode). To do so, the piezoelectric transducer may transform a pressurewave into electrical voltage that is then converted (by a rectifier andsupercapacitor) into stored electrical energy.

In illustrative implementations, the absorptive state of backscattermodulation provides an opportunity to harvest energy from thetransmitter's downlink acoustic signal since the harvesting involves aconversion between mechanical and electrical energy. The backscatternode may employ the harvested energy to control the backscatter switch.This energy may also be used for powering an onboard microcontroller andfor powering various onboard sensors. To ensure maximum power transferand optimize energy harvesting in the absorptive state, the analogfront-end of the backscatter node may employ an impedance matchingnetwork. This matching circuit may match the input electrical impedanceZ_(L) of the electrical load to the complex conjugate of the outputelectrical impedance Z_(s) of the piezoelectric source. Also, the systemmay employ a pulse width modulation (PWM) scheme on the downlink. ThePWM may be decoded using simple envelope detection, thus minimizingpower consumption during backscatter. Also, the PWM may provide ampleopportunities for energy harvesting.

As noted above, when the piezoelectric material is in the reflectivestate, the two terminals of the piezoelectric transducer may be shorted(i.e., Z_(L)=0), causing incident acoustic wave to be entirely (or to alarge extent) reflected. Thus, to maximize the SNR, the reflected powerin the absorptive state may be minimized. This may achieved by settingZ_(L)=Z_(s)*. Notice that this is the same impedance that maximizes theenergy transfer as discussed above.

An electrical signal outputted by the piezoelectric transducer may bedecoded to read a query or other communication encoded by a downlinkacoustic signal

In some implementations of this invention, using sound signals in theacoustic frequency range (1 Hz to 20 kHz) is highly advantageous. Thisis because, for sound, attenuation tends to increase as frequencyincreases. Ultrasound is typically not practical for transmission ofsignals over long distances, because the high frequency of ultrasoundcauses too rapid attenuation of the signal. In some implementations ofthis invention, the acoustic signals are in an upper portion (e.g., 10kHz to 20 kHz) of the acoustic range of frequencies.

In some cases, the acoustic backscatter communication system employs aprotocol similar to that used for RFIDs. For instance, an acoustictransmitter may transmit a query on the downlink which comprises apreamble, destination address, and payload. Likewise, the uplinkbackscatter packet may comprise a preamble, a header, and a payload. Theuplink payload may include sensor readings from one or more sensors.

In some cases, a single backscatter node operates at only a singlepre-defined resonant frequency. Alternatively, a backscatter node may beable to change its own resonant frequency. To do so, the backscatternode may include multiple matching circuits, each of which has adifferent resonant frequency. The backscatter node may tune its ownresonant frequency by switching which of these onboard matching circuitsis being used. Alternatively, a backscatter node may tune its ownresonant frequency by changing the state of one or more switches in amatching circuit onboard the node.

Each backscatter node may include one or more sensors.

In some cases: (a) a communication system includes multiple acousticbackscatter nodes; (b) each backscatter node in the system includes oneor more sensors; and (c) the backscatter networking enables multiplebackscatter nodes to concurrently transmit sensor readings at differentacoustic frequencies

Prototype

The following 23 paragraphs describe a prototype of this invention.

In this prototype, the acoustic transducer includes a piezoelectriccylinder with an in-air resonance frequency of 17 kHz, a radius of 2.5cm, and a length of 4 cm. The cylinder vibrates radially making itomnidirectional in the horizontal plane.

In this prototype, the acoustic transducer is air-backed and end-capped.The piezoelectric resonator is encapsulated with a polymer. End-capsseal off the top and bottom of the cylinder. The encapsulation andend-caps: (a) insulate the electrodes from water (preventing it fromshorting the electrodes) and (b) prevent water from flowing inside thecylinder and loading its resonance. This air-backed, end-cappedtransducer performs highly efficient electromechanical conversion.

FIG. 2 shows an exploded view of a piezoelectric transducer that is usedin this prototype. In FIG. 2, the piezoelectric transducer 200 includesa piezoelectric cylinder 205, washers 204, 206, a base 207, an end cap203, a bolt 202, and a polyurethane encasement 201.

In this prototype, two wires are soldered to the two electrodes of thepiezoelectric ceramic (i.e., the inner and outer surfaces of thecylinder). Washers (e.g., 204, 206) enable the cylinder to vibratefreely without being loaded by the end-caps. The setup is held tightusing a screw and locking nut, then placed inside the mold.

In this prototype, the transducer is encapsulated in polyurethane,specifically a polyurethane WC-575A mixture. This material is desirabledue to its transparency and because its acoustic impedance maximizes theenergy transfer between water and the piezoelectric material. Thepolyurethane encapsulation is fabricated as follows: The components ofthe polyurethane are mixed and then the resulting mixture is placedinside a vacuum tank to extract any moisture or residual air bubblesfrom the mixture. Then the prepared polyurethane is poured into acylindrical mold and left to pot for 12 hours in a pressure chamber at60 psi (4 atm). Once the potting is done, the mold is removed and marineepoxy is added to seal any remaining holes and ensure that water doesnot leak into the transducer.

In this prototype, the hardware is entirely battery-free and isfabricated on a two-layer printed circuit board (PCB). The hardwareperforms: (a) backscatter communication for uplink; (b) energyharvesting; (c) receiving and decoding for downlink signals; and (d) andinterfacing with peripherals. Thus, the hardware provides ageneral-purpose and extensible platform for battery-free underwatersensing and communication.

In this prototype, an acoustic backscatter node includes the hardwareshown in FIG. 3. In FIG. 3, backscatter node 300 includes: (a) apiezoelectric transducer 301; (b) switches 302 that control the loadimpedance to enable backscatter communication, (c) a matching network303, (c) a rectifying circuit 304 that converts AC to DC voltage andpassively amplifies the voltage, (d) a supercapacitor 305 that storesenergy from the rectified DC voltage, (e) a pull-down transistor 306 forimproving the SNR (signal-to-noise ratio) of the downlink signals (i.e.,the acoustic signals that travel from the acoustic speakers to thebackscatter node), a voltage regulator 307, and a microcontroller 308.For instance, switches 302 may comprise transistors.

In this prototype, the backscatter node has a differential analogfront-end (including every element shown in FIG. 3, except themicrocontroller).

In this prototype, a piezoelectric resonator provides a differentialoutput (rather than a single-ended output with a ground). Hence, theanalog front-end adopts a differential design as can be seen in themirrored architecture in FIG. 3, where the upper and lower portion ofthe energy harvesting and backscatter units are mirror images of eachother.

In this prototype, to enable backscatter communication, two transistorsare inserted in series between the two terminals of the piezoelectricdevice. The middle junction of the two transistors is connected toground, enabling symmetric backscatter and maximizing the SNR of thebackscattered signal. The transistors act as switches to enable togglingthe piezoelectric interface between reflective and non-reflectivestages, when they are operating in short-circuit and open-circuit modes,respectively. The gates of the transistors are controlled by themicrocontroller. The series configuration (in which the two transistorsare in series) enables controlling the switches at a lowergate-to-source voltage (V_(GS)) since the source is always at ground.This allows the microcontroller to switch between the two states at alow threshold voltage.

In this prototype, a multi-stage rectifier and a storage capacitor areused for harvesting electrical energy. The multi-stage rectifiertransforms the alternating electrical signal coming from the transducerinto a DC voltage by passing it through diodes and capacitors. Themulti-stage rectifier passively amplifies the voltage to a levelsufficient to activate the digital components of the circuit design. Therectified DC charge is stored in a 1000 microFarad supercapacitor.

In this prototype, load-matching is employed to achieve efficient energyharvesting. Specifically, during energy harvesting, the complexconjugate of the output impedance Z_(s) of the piezoelectric transducermay be matched to the input impedance Z_(L) of the load, that isZ_(L)=Z_(s)*. An impedance matching network 303 (which includes aninductor and a capacitor) may be inserted between the piezoelectrictransducer and the rectifier.

In this prototype, an energy-harvesting supercapacitor is connected to alow-dropout (LDO) voltage regulator, the output of which is 1.8 V. Thevoltage regulator drives the digital components of the circuit, ensuringthey are not damaged or operated in an unsteady mode.

In this prototype, the backscatter node employs envelope detection todecode data in the modulated acoustic signals. The downlinkcommunication signal (from acoustic speaker to backscatter node) isencoded using PWM (pulse width modification), where a larger pulse widthcorresponds to a “1” bit and a shorter pulse width corresponds to a “0”bit. In order to decode these pulses, the backscatter node performs edgedetection to identify the bit (pulse) boundaries and durations.

In this prototype, a Schmitt trigger discards small amplitude changes involtage due to noise and discretizes the output into two main voltagelevels: high and low. A level shifter scales the voltage levels toproperly condition them as inputs to a microcontroller.

In this prototype, a pull-down transistor improves both the energyharvesting efficiency and the decodability of the downlink signal.Specifically, the pull-down transistor acts as an open-circuit in thecold-start phase (i.e., when the supercapacitor is charging) to ensurethat all the incoming energy flows to the capacitor enabling fastcharging. Once the capacitor has enough voltage to power on the voltageregulator and the microcontroller (MCU), the MCU applies a voltage onthe pull-down transistor changing it to a closed circuit. While thisleaks some of the energy to ground, it also maximizes the differencebetween the high and low voltage levels at the input to the Schmitttrigger, thus improving the SNR for decoding the downlink PWM signal.

In this prototype, an ultra-low power microcontroller reduces powerconsumption. The microcontroller can operate with a supply voltage aslow as 1.8V and consumes less than 230 microamps at 1.8V in active modeand 0.5 microamps in low power mode (LMP3) with just one active clockusing a crystal oscillator operating at 32.8 kHz.

In this prototype, the microcontroller powers up. The microcontrollermay prepare to receive and decode a downlink command by enablinginterrupts and initializing a timer to detect a falling edge at its pinwhich is connected to the output of the level shifter; then, it entersLMP3 mode. A falling edge at the microcontroller's input may raise aninterrupt waking up the microcontroller, which enters active mode tocompute the time interval between every edge to decode bit “0” or “1” ofthe query, before going back to low-power mode.

In this prototype, after the microcontroller successfully decodesdownlink signals from the acoustic projector, the microcontroller mayprepare for backscatter. The microcontroller: (a) may switch the timerto continuous mode to enable controlling the switch at the backscatterfrequency; and (b) may employ FM0 encoding. An output pin of themicrocontroller is connected to the two switching transistors enablingthem to toggle the transducer between reflective and non-reflectivestates.

In this prototype, the microcontroller may also communicate with analogand digital peripheral sensors. The ADC pin is used for sampling analogsensors and the I²C protocol is used to communicate with digitalsensors.

In this prototype, each acoustic transmitter (projector) is connected tothe output audio jack of a personal computer through a two-channel 750 Wpower amplifier. The projector is configured to transmit signals atdifferent center frequencies between 12 kHz and 18 kHz. For eachdifferent configuration, a different matching circuit may be employed tooptimize power transfer between the power amplifier and the transducer.The transmitted signal is generated using MATLAB® software and employsPWM where the “1” bit is twice as long as the “0” bit. The transmitter'sdownlink query includes a 9-bit preamble. The transmitter packet mayalso include commands for the PAB backscatter node such as settingbackscatter link frequency, switching its resonance mode, or requestingcertain sensed data like pH, temperature, or pressure.

In this prototype, the microphone is a hydrophone, whose sensitivity is−180 dB re: 1V/μPa. The hydrophone is connected to the audio jack of apersonal computer. An Audacity® software package is used to record thereceived audio signals. The signals are processed offline using aMATLAB®-based decoder. The decoder identifies the different transmittedfrequencies on the downlink using FFT (fast Fourier transform) and peakdetection. The decoder then downconverts the signals to baseband bymultiplying each of them with its respective carrier frequency. Thereceiver then employs a Butterworth filter on each of the receivechannels to isolate the signal of interest and reduce interference fromconcurrent transmissions. Subsequently, the decoder performs standardpacket detection and carrier frequency offset (CFO) correction using thepreamble. The decoder then employs a maximum likelihood decoder todecode the FM0 decoded bits. It may also use CRC (cyclic redundancycheck) to perform a checksum on the received packets and requestretransmissions of corrupted packets. The communication system includesa separate transmitter (projector) and receiver (hydrophone). Hence, thereceiver observes a CFO due to the different oscillators.

In this prototype, the microcontroller in the backscatter node mayinterface with one or more sensors, such as sensors that measure pH,pressure or temperature.

The prototype described in the preceding 23 paragraphs is a non-limitingexample of this invention. This invention may be implemented in manyother ways.

Reflection States and Encoded Bits

In some implementations of this invention, one bit of data is encoded bya single acoustic reflection state or by a transition between tworeflection states. FIGS. 4A and 4B illustrate electrical circuits forencoding one bit of data per acoustic reflection state. In FIGS. 4A and4B, the electrical impedance of a load—and thus the acoustic reflectanceof piezoelectric transducer 401—is controlled by a switch (e.g.,transistor switch 402 or switch 403). The two positions of the switchcorrespond to two different reflection states of the backscatter node(e.g., high reflectance and low reflectance, respectively).

In some implementations, each acoustic reflection state is defined by anordered pair of amplitude and phase. In this implementations, two ormore bits of data are encoded by a single acoustic reflection state orby a transition between two reflection states. Each of the differentreflection states may be an ordered pair of amplitude and phase ofmodulated light that reflects from the backscatter node.

FIG. 5 shows an electrical circuit for encoding two or more bits of dataper acoustic reflection state. In FIG. 5, the acoustic reflectance ofpiezoelectric transducer 501 is controlled by changing electricalimpedance of an electrical load. In the example shown in FIG. 5, theelectrical impedance is controlled by a switch 502 which selects one ofa set of electrical paths 503 that each have a different electricalimpedance. These different electrical impedances may be created by acombination of one or more resistors, inductors and capacitors. In FIG.5, each of the different electrical impedances (Z₁, Z₂, Z₃, . . . Z_(N))correspond to a different ordered pair of amplitude and phase ofelectrical impedance. In actual practice, an equivalent circuit may beemployed, instead of the circuit shown in FIG. 5.

In FIG. 5, if N=4, then: (a) the electrical load may be switched to anyof four different electrical impedances, each of which corresponds to adifferent ordered pair of amplitude and phase; (b) the backscatter nodemay have any of four different reflection states; and (b) two bits areencoded by a single reflection state or by a single transition betweenreflection states.

In FIG. 5, if N=8, then: (a) the electrical load may be switched to anyof eight different electrical impedances, each of which corresponds to adifferent ordered pair of amplitude and phase; (b) the backscatter nodemay have any of eight different reflection states; and (b) three bitsare encoded by a single reflection state or by a single transitionbetween reflection states.

Concurrent Backscatter Signals

In some implementations, multiple backscatter nodes send acousticsignals concurrently, each at a different center frequency, therebyenabling higher data throughput.

In this approach, each of the backscatter nodes has a different acousticresonant frequency and reflects modulated acoustic signals at a centerfrequency that corresponds to this resonant frequency. The resonantfrequency of a backscatter node may be tuned by a circuit (e.g., an RLCcircuit) onboard the backscatter node.

In this approach, the acoustic signals may be sent in accordance with aMAC (medium access control) protocol that enables decoding of networkcollisions. In some cases, an FDMA (frequency-division multiple access)protocol is employed. For instance, in some cases: (a) each backscatternode has a slightly different resonance frequency; and (b) the differentbackscatter nodes occupy different bands of the acoustic frequencyspectrum, thereby facilitating FDMA. In some cases: (a) differentacoustic projectors transmit acoustic signals at different frequencies;and (b) each acoustic projector activates a different backscatter nodeoperating at the corresponding resonance frequency, thus facilitatingconcurrent multiple access. The microphone (e.g., a hydrophone) mayreceive all the reflected signals and apply software-based filters inorder to isolate and decode the colliding backscatter reflections. Thereceiving microphone (e.g., hydrophone) may employ a MIMO(multiple-input and multiple-output) decoder to deal with collisions.

In some cases, more than one microphone is employed to measure thebackscattered modulated signals. For instance, different microphones maybe employed to measure different frequency channels.

In some cases, concurrent communication at different acousticfrequencies is used to report readings from multiple sensors. Forinstance: (a) multiple backscatter nodes may have a different acousticresonant frequency; and (b) each backscatter node may reflect modulatedacoustic signals that encode data recorded by one or more sensors.

In some cases, each backscatter node includes (or is housed togetherwith and electrically connected to) one or more sensors.

In some cases, acoustic signals with different center frequencies areemployed, but the acoustic signals may still overlap to some extent andsignal collisions may still occur. To extract the different signals fromthe measurements taken by the microphone(s), one or more computers mayperform a MIMO decoder algorithm. For instance, the MIMO decoderalgorithm may be: (a) a linear MIMO decoder, such as a matched filter(MF) detector, linear zero-forcing (ZF) detector, linear minimummean-square error (MMSE) detector, linear maximumasymptotic-multiuser-efficiency (MAME) detector, linear weightedleast-squares (WLS) detector, linear minimum bit error rate (MBER)detector; (b) an interference-cancellation MIMO detector, such as asuccessive interference cancellation (SIC) detector, parallelinterference cancellation (PIC) detector, multistage interferencecancellation (MIC) detector, or decision-feedback detector (DFD), (c) atree-search MIMO detector, such as the sphere decoder (SD), (d)lattice-reduction (LR) MIMO detector, (e) a probabilistic dataassociation (PDA) MIMO detector, or (f) a semidefinite programmingrelaxation (SDPR) MIMO detector.

In some implementations, backscatter modulation is frequency-agnostic.Specifically, as long as an acoustic backscatter node powers up (due toa downlink signal within its resonance bandwidth), the node maybackscatter and modulate the reflections of all acoustic signalsimpinging on it, even those outside its resonance frequency. (Themodulation depth of backscattered signals—i.e., difference betweenreflected and absorbed power—may decrease as their frequency moves awayfrom resonance due to the degradation in impedance matching andefficiency). Thus, in some cases, even if a backscatter node is turnedto a first acoustic resonant frequency, the node may also backscattersignals at a second acoustic frequency, thus interfering with aconcurrent transmission at the second frequency.

To overcome this challenge, MIMO decoding may be employed. For instance,if two different acoustic backscatter nodes are tuned to two differentresonant frequencies, then a microphone receiver may measure thefollowing two signals:

y(f ₁)=h ₁(f ₁)x ₁ +h ₂(f ₁)x ₂

y(f ₂)=h ₁(f ₂)x ₁ +h ₂(f ₂)x ₂

where y is the received signal, f₁ and f₂ are the resonance frequencies,x₁ and x₂ are the backscattered signals reflected from the two differentnodes, and h₁ and h₂ are their corresponding frequency-selectivechannels.

In the example described in the preceding paragraph, a computer mayperform channel estimation, and then may invert the channel matrix anddecode the two signals using MIMO decoding. More generally, if Nacoustic backscatter nodes with N resonant frequencies reflect acousticsignals, then channel estimation may be performed, and an N×N channelmatrix may be inverted to decode the signals using a MIMO decoderalgorithm.

FIG. 6 illustrates a system in which acoustic backscatter is employed tocommunicate concurrently over different frequency channels. In FIG. 6,speakers 601, 602, and 603 transmit acoustic signals at different centerfrequencies. Backscatter nodes 611, 612, and 613 have different resonantfrequencies that are equal to the respective center frequencies at whichthe speakers are transmitting. These different resonant frequencies aredue to different tuning circuits onboard the backscatter nodes.Likewise, the modulated acoustic signals that reflect from backscatternodes 611, 612, and 613 have the same center frequencies as the centerfrequencies at which the respective transmitters are transmitting.

In FIG. 6, each backscatter node includes one or more sensors.

Specifically: (a) backscatter node 611 includes sensor 621; (b)backscatter node 612 includes sensors 622 and 623; and (c) backscatternode 613 includes sensors 624, 625 and 626. A wide variety of low-powersensors (e.g., pH, pressure, temperature and salinity sensors) may beemployed. In FIG. 6, the modulated acoustic signals that reflect fromeach backscatter node may (during certain time periods) encode sensorreadings taken by the one or more sensors that are included in thatnode. A single microphone (e.g., hydrophone) 620 may record themodulated acoustic signals from the different backscatter nodes. Acomputer 630 may control and interface with the transmitters andmicrophone, and may decode data that is encoded in the modulatedacoustic signals. For instance, computer 630 may decode the sensorreadings taken by the sensors in the backscatter nodes.

Near-Audio Ultrasonic Backscatter Communication

The discussion above focuses on backscatter communication in theacoustic range of frequencies (1 Hz to 20 kHz). However, this inventionis not limited to acoustic signals.

For instance, in some alternate implementations of this invention,backscatter communication occurs in what we call the “near-audioultrasonic” range of frequencies (greater than 20 kHz and less than orequal to 100 kHz). To describe these alternate implementations, eachreference herein to “acoustic” may be replaced by “near-audioultrasonic” (for instance, each time that the phrase “acoustic signal”occurs herein, it may be replaced by “near-audio ultrasonic signal”).

Computers

In illustrative implementations of this invention, a communicationsystem includes computers (e.g. 104, 120, 308, 630). These computers(e.g., servers, network hosts, client computers, integrated circuits,microcontrollers, controllers, microprocessors, field-programmable-gatearrays, personal computers, digital computers, driver circuits, oranalog computers) may be programmed or specially adapted to perform oneor more of the following tasks: (1) to control the operation of, orinterface with, hardware components of an acoustic backscattercommunication network, including any speakers, microphones, backscatternodes, switches, and sensors; (2) to encode or decode uplink messages;(3) to encode or decode downlink messages; (4) to control modulation ofacoustic signals that reflect from a backscatter node; (5) to receivedata from, control, or interface with one or more sensors; (6) toperform any other calculation, computation, program, algorithm, orcomputer function described or implied herein; (7) to receive signalsindicative of human input; (8) to output signals for controllingtransducers for outputting information in human perceivable format; (9)to process data, to perform computations, and to execute any algorithmor software; and (10) to control the read or write of data to and frommemory devices (tasks 1-10 of this sentence being referred to herein asthe “Computer Tasks”).

In exemplary implementations, one or more computers are programmed toperform any and all calculations, computations, programs, algorithms,computer functions and computer tasks described or implied herein. Forexample, in some cases: (a) a machine-accessible medium has instructionsencoded thereon that specify steps in a software program; and (b) thecomputer accesses the instructions encoded on the machine-accessiblemedium, in order to determine steps to execute in the program. Inexemplary implementations, the machine-accessible medium may comprise atangible non-transitory medium. In some cases, the machine-accessiblemedium comprises (a) a memory unit or (b) an auxiliary memory storagedevice. For example, in some cases, a control unit in a computer fetchesthe instructions from memory.

In illustrative implementations, one or more computers execute programsaccording to instructions encoded in one or more tangible,non-transitory computer-readable media. For example, in some cases,these instructions comprise instructions for a computer to perform anycalculation, computation, program, algorithm, or computer functiondescribed or implied herein. For instance, in some cases, instructionsencoded in a tangible, non-transitory, computer-accessible mediumcomprise instructions for a computer to perform the Computer Tasks.

Computer Readable Media

In some implementations, this invention comprises one or more computersthat are programmed to perform one or more of the Computer Tasks.

In some implementations, this invention comprises one or more tangible,machine readable media, with instructions encoded thereon for one ormore computers to perform one or more of the Computer Tasks. In someimplementations, these one or more media are not transitory waves andare not transitory signals.

In some implementations, this invention comprises participating in adownload of software, where the software comprises instructions for oneor more computers to perform one or more of the Computer Tasks. Forinstance, the participating may comprise (a) a computer providing thesoftware during the download, or (b) a computer receiving the softwareduring the download.

Definitions

The terms “a” and “an”, when modifying a noun, do not imply that onlyone of the noun exists. For example, a statement that “an apple ishanging from a branch”: (i) does not imply that only one apple ishanging from the branch; (ii) is true if one apple is hanging from thebranch; and (iii) is true if multiple apples are hanging from thebranch.

“AC” means alternating current.

As used herein, “acoustic backscatter node” means a device that isconfigured to reflect one or more acoustic signals. A human is not an“acoustic backscatter node”, as that term is used herein.

As used herein, “acoustic microphone” means a microphone that isconfigured to measure one or more acoustic signals.

As used herein, “acoustic power” means power of an acoustic signal.

As used herein, “acoustic reflectance” of an object is the fraction ofacoustic power reflected from the object. Thus, acoustic reflectance isa measure of the effectiveness of the object in reflecting acousticsignals.

As used herein, “acoustic signal” means a signal that comprises soundand that has a peak frequency greater than or equal to 1 Hertz and lessthan or equal to 20,000 Hertz.

As used herein, “acoustic resonant frequency” means a resonant frequencythat is greater than or equal to 1 Hertz and less than or equal to20,000 Hertz.

As used herein, “acoustic transmitter” means a transmitter that isconfigured to transmit one or more acoustic signals. A human is not an“acoustic transmitter”, as that term is used herein.

To compute “based on” specified data means to perform a computation thattakes the specified data as an input.

The term “comprise” (and grammatical variations thereof) shall beconstrued as if followed by “without limitation”. If A comprises B, thenA includes B and may include other things.

Each of the following is a non-limiting example of a “computer”, as thatterm is used herein: (a) a digital computer; (b) an analog computer; (c)a computer that performs both analog and digital computations; (d) amicrocontroller; (e) a microprocessor; (f) a controller; (g) a tabletcomputer; (h) a notebook computer; (i) a laptop computer, (j) a personalcomputer; (k) a mainframe computer; and (1) a quantum computer. However,a human is not a “computer”, as that term is used herein.

“Computer Tasks” is defined above.

“DC” means direct current.

“Defined Term” means a term or phrase that is set forth in quotationmarks in this Definitions section.

As used herein, “downlink” acoustic signal means an acoustic signal thattravels from an acoustic transmitter to an acoustic backscatter node.

For an event to occur “during” a time period, it is not necessary thatthe event occur throughout the entire time period. For example, an eventthat occurs during only a portion of a given time period occurs “during”the given time period.

The term “e.g.” means for example.

The fact that an “example” or multiple examples of something are givendoes not imply that they are the only instances of that thing. Anexample (or a group of examples) is merely a non-exhaustive andnon-limiting illustration.

Unless the context clearly indicates otherwise: (1) a phrase thatincludes “a first” thing and “a second” thing does not imply an order ofthe two things (or that there are only two of the things); and (2) sucha phrase is simply a way of identifying the two things, so that theyeach may be referred to later with specificity (e.g., by referring to“the first” thing and “the second” thing later). For example, if adevice has a first socket and a second socket, then, unless the contextclearly indicates otherwise, the device may have two or more sockets,and the first socket may occur in any spatial order relative to thesecond socket. A phrase that includes a “third” thing, a “fourth” thingand so on shall be construed in like manner.

“For instance” means for example.

To say a “given” X is simply a way of identifying the X, such that the Xmay be referred to later with specificity. To say a “given” X does notcreate any implication regarding X. For example, to say a “given” X doesnot create any implication that X is a gift, assumption, or known fact.

“Herein” means in this document, including text, specification, claims,abstract, and drawings.

As used herein: (1) “implementation” means an implementation of thisinvention; (2) “embodiment” means an embodiment of this invention; (3)“case” means an implementation of this invention; and (4) “use scenario”means a use scenario of this invention.

The term “include” (and grammatical variations thereof) shall beconstrued as if followed by “without limitation”.

As used herein, “near-audio ultrasonic backscatter node” means a devicethat is configured to reflect one or more near-audio ultrasonic signals.A human is not a “near-audio ultrasonic backscatter node”, as that termis used herein.

As used herein, “near-audio ultrasonic microphone” means a microphonethat is configured to measure one or more near-audio ultrasonic signals.

As used herein, “near-audio ultrasonic power” means power of anear-audio ultrasonic signal.

As used herein, “near-audio ultrasonic reflectance” of an object is thefraction of near-audio ultrasonic power reflected from the object. Thus,near-audio ultrasonic reflectance is a measure of the effectiveness ofthe object in reflecting near-audio ultrasonic signals.

As used herein, “near-audio ultrasonic signal” means a signal thatcomprises sound and that has a peak frequency greater than 20,000 Hertzand less than or equal to 100,000 Hertz.

As used herein, “near-audio ultrasonic resonant frequency” means aresonant frequency that is greater than 20,000 Hertz and less than orequal to 100,000 Hertz.

As used herein, “near-audio ultrasonic transmitter” means a transmitterthat is configured to transmit one or more near-audio ultrasonicsignals. A human is not a “near-audio ultrasonic transmitter”, as thatterm is used herein.

Unless the context clearly indicates otherwise, “or” means and/or. Forexample, A or B is true if A is true, or B is true, or both A and B aretrue. Also, for example, a calculation of A or B means a calculation ofA, or a calculation of B, or a calculation of A and B.

As used herein, the term “set” does not include a group with noelements.

“SNR” means signal-to-noise ratio.

Unless the context clearly indicates otherwise, “some” means one ormore.

As used herein, a “subset” of a set consists of less than all of theelements of the set.

The term “such as” means for example.

To say that a machine-readable medium is “transitory” means that themedium is a transitory signal, such as an electromagnetic wave.

As used herein, “uplink” acoustic signal means an acoustic signal thattravels from an acoustic backscatter node to an acoustic microphone.

Except to the extent that the context clearly requires otherwise, ifsteps in a method are described herein, then the method includesvariations in which: (1) steps in the method occur in any order orsequence, including any order or sequence different than that describedherein; (2) any step or steps in the method occur more than once; (3)any two steps occur the same number of times or a different number oftimes during the method; (4) one or more steps in the method are done inparallel or serially; (5) any step in the method is performediteratively; (6) a given step in the method is applied to the same thingeach time that the given step occurs or is applied to a different thingeach time that the given step occurs; (7) one or more steps occursimultaneously; or (8) the method includes other steps, in addition tothe steps described herein.

Headings are included herein merely to facilitate a reader's navigationof this document. A heading for a section does not affect the meaning orscope of that section.

This Definitions section shall, in all cases, control over and overrideany other definition of the Defined Terms. The Applicant or Applicantsare acting as his, her, its or their own lexicographer with respect tothe Defined Terms. For example, the definitions of Defined Terms setforth in this Definitions section override common usage and any externaldictionary. If a given term is explicitly or implicitly defined in thisdocument, then that definition shall be controlling, and shall overrideany definition of the given term arising from any source (e.g., adictionary or common usage) that is external to this document. If thisdocument provides clarification regarding the meaning of a particularterm, then that clarification shall, to the extent applicable, overrideany definition of the given term arising from any source (e.g., adictionary or common usage) that is external to this document. Unlessthe context clearly indicates otherwise, any definition or clarificationherein of a term or phrase applies to any grammatical variation of theterm or phrase, taking into account the difference in grammatical form.For example, the grammatical variations include noun, verb, participle,adjective, and possessive forms, and different declensions, anddifferent tenses.

Variations

This invention may be implemented in many different ways. Here are somenon-limiting examples:

In some implementations, this invention is a system comprising: (a) oneor more acoustic transmitters; (b) a set of one or more acousticbackscatter nodes; and (c) one or more acoustic microphones; wherein (i)the one or more acoustic transmitters are configured to transmitdownlink acoustic signals that travel to the set, (ii) each particularacoustic backscatter node in the set is configured to performmodulation, in such a way as to modulate amplitude and/or phase of soundthat reflects from the particular node, thereby producing modulateduplink acoustic signals that encode data and that travel from theparticular node to the one or more acoustic microphones, and (iii) atleast one of the one or more acoustic microphones is configured to takemeasurements of the modulated uplink acoustic signals. In some cases,each particular acoustic backscatter node in the set: (a) includes apiezoelectric transducer and a circuit; and (b) is configured to changeelectrical impedance of the circuit and to thereby change acousticreflectance of the piezoelectric transducer. In some cases: (a) thedownlink acoustic signals include a set of acoustic signals, each at adifferent center frequency; and (b) the one or more transmitters areconfigured to transmit the set of acoustic signals at the differentcenter frequencies concurrently. In some cases, at least one acousticbackscatter node in the set includes one or more sensors. In some cases,each acoustic backscatter node in the set: (a) includes a piezoelectrictransducer; (b) also includes multiple circuits that each have adifferent resonant frequency; and (c) is configured to switch which ofthe matching circuits forms an electrical loop with the piezoelectrictransducer. In some cases: (a) the set of nodes includes multipleacoustic backscatter nodes; and (b) the multiple acoustic backscatternodes are configured to perform the modulation in accordance with amedium access control protocol. In some cases: (a) the downlink acousticsignals include a group of acoustic signals, each at a different centerfrequency; (b) the one or more transmitters are configured to transmitthe group of acoustic signals at the different center frequencies; (c)the set of nodes includes multiple acoustic backscatter nodes; and (d)the multiple acoustic backscatter nodes are configured to perform themodulation in accordance with a frequency-division multiple accessprotocol. In some cases, each acoustic backscatter node in the setincludes hardware that is configured to harvest electrical energy fromthe downlink acoustic signals, which hardware includes: (a) apiezoelectric transducer that is configured to convert pressure wavesinto an electrical signal, which pressure waves occur in the transducerand are induced by the downlink acoustic signals; (b) a rectifier thatis configured to convert the electrical signal into direct currentvoltage; and (c) one or more capacitors that are configured to storeelectrical energy. In some cases, each acoustic backscatter node in theset includes a piezoelectric transducer and a circuit that has an inputelectrical impedance, which input impedance matches the complexconjugate of an output electrical impedance of the piezoelectrictransducer. In some cases, each bit of data that is encoded by themodulated uplink acoustic signals corresponds to a reflection state, orto a transition between two reflection states, of an acousticbackscatter node in the set. In some cases, each acoustic backscatternode in the set: (a) includes a piezoelectric transducer; and (b) isconfigured to perform the modulation in such a way that (i) thepiezoelectric transducer undergoes different reflection states atdifferent times during the modulation, which reflection states aremembers of a group of reflection states that are each determined byamplitude and phase of the complex conjugate of an input electricalimpedance of an electrical load that is electrically connected to thepiezoelectric transducer, and (iii) two or more bits of the data areencoded by a reflection state in the group, or by a transition betweenreflection states in the group. In some cases: (a) the set of nodesincludes multiple acoustic backscatter nodes; (b) each of the multipleacoustic backscatter nodes has a different resonant frequency; and (c)the system includes one or more computers that are programmed to performa multiple-input and multiple-output algorithm. Each of the casesdescribed above in this paragraph is an example of the system describedin the first sentence of this paragraph, and is also an example of anembodiment of this invention that may be combined with other embodimentsof this invention.

In some implementations, this invention is a system comprising: (a) oneor more acoustic transmitters; (b) a set of one or more acousticbackscatter nodes; and (c) one or more acoustic microphones; wherein (i)the one or more acoustic transmitters are configured to transmitdownlink acoustic signals that travel through a liquid medium to theset, (ii) each particular acoustic backscatter node in the set (A)includes a piezoelectric transducer, and (B) is configured to performmodulation, in such a way as to modulate amplitude and/or phase of soundthat reflects from the particular node, thereby producing modulateduplink acoustic signals that encode data and that travel through theliquid medium from the particular node to the one or more acousticmicrophones, and (iii) at least one of the one or more acousticmicrophones is configured to take measurements of the modulated uplinkacoustic signals. In some cases, the liquid medium is ocean water. Insome cases: (a) the downlink acoustic signals include a set of acousticsignals, each at a different center frequency; and (b) the one or moretransmitters are configured to transmit the set of acoustic signals atthe different center frequencies concurrently. In some cases, the set ofnodes includes multiple acoustic backscatter nodes that each: (a)include one or more switches; (b) have an acoustic resonant frequency;and (c) are configured to modify the acoustic resonant frequency bychanging a state of one or more of the switches. Each of the casesdescribed above in this paragraph is an example of the system describedin the first sentence of this paragraph, and is also an example of anembodiment of this invention that may be combined with other embodimentsof this invention.

In some implementations, this invention is a system comprising: (a) oneor more acoustic transmitters; (b) a set of one or more acousticbackscatter nodes; and (c) one or more acoustic microphones; wherein (i)the one or more acoustic transmitters are configured to transmitdownlink acoustic signals that travel through a solid medium to the set,(ii) each particular acoustic backscatter node in the set (A) includes apiezoelectric transducer, and (B) is configured to perform modulation,in such a way as to modulate amplitude and/or phase of sound thatreflects from the particular node, thereby producing modulated uplinkacoustic signals that encode data and that travel through the solidmedium from the particular node to the one or more acoustic microphones,and (iii) at least one of the one or more acoustic microphones isconfigured to take measurements of the modulated uplink acousticsignals. In some cases, the solid material includes metal. In somecases, the solid material includes wood. In some cases, the set of nodesincludes multiple acoustic backscatter nodes that each: (a) include oneor more switches; (b) have an acoustic resonant frequency; and (c) areconfigured to modify the acoustic resonant frequency by changing a stateof one or more of the switches. Each of the cases described above inthis paragraph is an example of the system described in the firstsentence of this paragraph, and is also an example of an embodiment ofthis invention that may be combined with other embodiments of thisinvention.

Each description herein (or in the Provisional) of any method, apparatusor system of this invention describes a non-limiting example of thisinvention. This invention is not limited to those examples, and may beimplemented in other ways.

Each description herein (or in the Provisional) of any prototype of thisinvention describes a non-limiting example of this invention. Thisinvention is not limited to those examples, and may be implemented inother ways.

Each description herein (or in the Provisional) of any implementation,embodiment or case of this invention (or any use scenario for thisinvention) describes a non-limiting example of this invention. Thisinvention is not limited to those examples, and may be implemented inother ways.

Each Figure, diagram, schematic or drawing herein (or in theProvisional) that illustrates any feature of this invention shows anon-limiting example of this invention. This invention is not limited tothose examples, and may be implemented in other ways.

The above description (including without limitation any attacheddrawings and figures) describes illustrative implementations of theinvention. However, the invention may be implemented in other ways. Themethods and apparatus which are described herein are merely illustrativeapplications of the principles of the invention. Other arrangements,methods, modifications, and substitutions by one of ordinary skill inthe art are also within the scope of the present invention. Numerousmodifications may be made by those skilled in the art without departingfrom the scope of the invention. Also, this invention includes withoutlimitation each combination and permutation of one or more of the items(including any hardware, hardware components, methods, processes, steps,software, algorithms, features, and technology) that are describedherein.

What is claimed:
 1. A system comprising: (a) one or more acoustictransmitters; (b) a set of one or more acoustic backscatter nodes; and(c) one or more acoustic microphones; wherein (i) the one or moreacoustic transmitters are configured to transmit downlink acousticsignals that travel to the set, (ii) each particular acousticbackscatter node in the set is configured to perform modulation, in sucha way as to modulate amplitude and/or phase of sound that reflects fromthe particular node, thereby producing modulated uplink acoustic signalsthat encode data and that travel from the particular node to the one ormore acoustic microphones, and (iii) at least one of the one or moreacoustic microphones is configured to take measurements of the modulateduplink acoustic signals.
 2. The system of claim 1, wherein eachparticular acoustic backscatter node in the set: (a) includes apiezoelectric transducer and a circuit; and (b) is configured to changeelectrical impedance of the circuit and to thereby change acousticreflectance of the piezoelectric transducer.
 3. The system of claim 1,wherein: (a) the downlink acoustic signals include a set of acousticsignals, each at a different center frequency; and (b) the one or moretransmitters are configured to transmit the set of acoustic signals atthe different center frequencies concurrently.
 4. The system of claim 1,wherein at least one acoustic backscatter node in the set includes oneor more sensors.
 5. The system of claim 1, wherein each acousticbackscatter node in the set: (a) includes a piezoelectric transducer;(b) also includes multiple circuits that each have a different resonantfrequency; and (c) is configured to switch which of the matchingcircuits forms an electrical loop with the piezoelectric transducer. 6.The system of claim 1, wherein: (a) the set of nodes includes multipleacoustic backscatter nodes; and (b) the multiple acoustic backscatternodes are configured to perform the modulation in accordance with amedium access control protocol.
 7. The system of claim 1, wherein: (a)the downlink acoustic signals include a group of acoustic signals, eachat a different center frequency; (b) the one or more transmitters areconfigured to transmit the group of acoustic signals at the differentcenter frequencies; (c) the set of nodes includes multiple acousticbackscatter nodes; and (d) the multiple acoustic backscatter nodes areconfigured to perform the modulation in accordance with afrequency-division multiple access protocol.
 8. The system of claim 1,wherein each acoustic backscatter node in the set includes hardware thatis configured to harvest electrical energy from the downlink acousticsignals, which hardware includes: (a) a piezoelectric transducer that isconfigured to convert pressure waves into an electrical signal, whichpressure waves occur in the transducer and are induced by the downlinkacoustic signals; (b) a rectifier that is configured to convert theelectrical signal into direct current voltage; and (c) one or morecapacitors that are configured to store electrical energy.
 9. The systemof claim 1, wherein each acoustic backscatter node in the set includes apiezoelectric transducer and a circuit that has an input electricalimpedance, which input impedance matches the complex conjugate of anoutput electrical impedance of the piezoelectric transducer.
 10. Thesystem of claim 1, wherein each bit of data that is encoded by themodulated uplink acoustic signals corresponds to a reflection state, orto a transition between two reflection states, of an acousticbackscatter node in the set.
 11. The system of claim 1, wherein eachacoustic backscatter node in the set: (a) includes a piezoelectrictransducer; and (b) is configured to perform the modulation in such away that (i) the piezoelectric transducer undergoes different reflectionstates at different times during the modulation, which reflection statesare members of a group of reflection states that are each determined byamplitude and phase of the complex conjugate of an input electricalimpedance of an electrical load that is electrically connected to thepiezoelectric transducer, and (iii) two or more bits of the data areencoded by a reflection state in the group, or by a transition betweenreflection states in the group.
 12. The system of claim 1, wherein: (a)the set of nodes includes multiple acoustic backscatter nodes; (b) eachof the multiple acoustic backscatter nodes has a different resonantfrequency; and (c) the system includes one or more computers that areprogrammed to perform a multiple-input and multiple-output algorithm.13. A system comprising: (a) one or more acoustic transmitters; (b) aset of one or more acoustic backscatter nodes; and (c) one or moreacoustic microphones; wherein (i) the one or more acoustic transmittersare configured to transmit downlink acoustic signals that travel througha liquid medium to the set, (ii) each particular acoustic backscatternode in the set (A) includes a piezoelectric transducer, and (B) isconfigured to perform modulation, in such a way as to modulate amplitudeand/or phase of sound that reflects from the particular node, therebyproducing modulated uplink acoustic signals that encode data and thattravel through the liquid medium from the particular node to the one ormore acoustic microphones, and (iii) at least one of the one or moreacoustic microphones is configured to take measurements of the modulateduplink acoustic signals.
 14. The system of claim 13, wherein the liquidmedium is ocean water.
 15. The system of claim 13, wherein: (a) thedownlink acoustic signals include a set of acoustic signals, each at adifferent center frequency; and (b) the one or more transmitters areconfigured to transmit the set of acoustic signals at the differentcenter frequencies concurrently.
 16. The system of claim 13, wherein theset of nodes includes multiple acoustic backscatter nodes that each: (a)include one or more switches; (b) have an acoustic resonant frequency;and (c) are configured to modify the acoustic resonant frequency bychanging a state of one or more of the switches.
 17. A systemcomprising: (a) one or more acoustic transmitters; (b) a set of one ormore acoustic backscatter nodes; and (c) one or more acousticmicrophones; wherein (i) the one or more acoustic transmitters areconfigured to transmit downlink acoustic signals that travel through asolid medium to the set, (ii) each particular acoustic backscatter nodein the set (A) includes a piezoelectric transducer, and (B) isconfigured to perform modulation, in such a way as to modulate amplitudeand/or phase of sound that reflects from the particular node, therebyproducing modulated uplink acoustic signals that encode data and thattravel through the solid medium from the particular node to the one ormore acoustic microphones, and (iii) at least one of the one or moreacoustic microphones is configured to take measurements of the modulateduplink acoustic signals.
 18. The system of claim 17, wherein the solidmaterial includes metal.
 19. The system of claim 17, wherein the solidmaterial includes wood.
 20. The system of claim 17, wherein the set ofnodes includes multiple acoustic backscatter nodes that each: (a)include one or more switches; (b) have an acoustic resonant frequency;and (c) are configured to modify the acoustic resonant frequency bychanging a state of one or more of the switches.